Update on Malignant Mesothelioma

Mesotheliomas are uncommon in the United States, with an incidence of about 3,000 new cases per year (or a risk of about 11 per million Americans per year). Incidence and mortality, however, are probably underestimated. Most are associated with asbestos, although some have arisen in ports of prior radiation, and a reported association with simian virus (SV)40 remains controversial. About 85% of mesotheliomas arise in the pleura, about 9% in the peritoneum, and a small percentage in the pericardium or tunica vaginalis testis. The histology of about half of mesotheliomas is epithelial (tubular papillary), with the remainder sarcomatous or mixed. Multicystic mesotheliomas and well-differentiated papillary mesotheliomas are associated with long survival in the absence of treatment and should be excluded from clinical trials intended for the usual rapidly lethal histologic variants of the disease. The median survival is under a year, although longer median survivals for selected patients, particularly those with epithelial histology, have been reported in some combined-modality studies. Recent randomized trials have shown significant improvement in time to progression and survival for the addition of new antifolates to platinum-based chemotherapy.

Mesotheliomas are uncommon in the United States, with an incidence of about 3,000 new cases per year (or a risk of about 11 per million Americans per year, Figure 1).[1] A fundamental issue for research in mesothelioma is the lack of accurate statistics. Available statistics probably underestimate mortality. The pathologic diagnosis is often difficult and may be inaccurate as well. Death certificates often cite lung cancer or heart failure as the cause of death. The disease is rare and thus physicians are inexperienced, leading to a delay in diagnosis and wide vari ations in care. Except at the few centers with research and treatment programs, few tissues are available for research studies. Epidemiology About 50% to 70% of mesotheliomas are associated with exposure to asbestos. In Turkey, mesothelioma, endemic in some areas, is associated with an asbestiform mineral, erionite, in white soil used as a whitewash.[2,3] Familial susceptibility may also be a factor.[4,5] In addition, mesothelio delivered for a prior malignancy or after extravasation of thorium dioxide contrast (Thorotrast).[6-8] Smoking and asbestos exposure substantially increases the risk of lung cancer, but smoking is not implicated in the etiology of mesotheliomas. Of uncertain significance, simian virus (SV)40 DNA fragments have been detected in mesotheliomas in some but not other studies.[9,10] SV40-contaminated polio vaccine was distributed in the 1950s. SV40 sequences have been found in microdis sected tumor cells but not in stroma in a substantial fraction of US mesothelioma patients. SV40 can cause mesothelioma in hamsters. The oncoproteins of SV40 bind to Rb and p53. A causal link in human mesothelioma, however, remains controversial.[9,10] AsbestosAsbestos, a silicate mineral of magnesium, calcium, and iron, occurs as fibrous rock, which is mined and milled. Asbestos fibers are either needle-like (amphiboles including amosite, tremolite and crocidolite) or spiral (chrysotile). Risk of mesothelioma varies by asbestos type broadly in the ratio of 1:100:500 for chrysotile, amosite, and crocidolite, respectively.[ 11] Although chrysotile is associated with the lowest risk, it nevertheless appears to result in some risk in animals and humans.[12] The word "asbestos" is derived from a Greek root for inextinguishable or indestructible. Asbestos use is documented for more than 6,000 years. Persians burned bodies in asbestos cloth to preserve the ashes. Pliny the Elder observed that the asbestos quarry slaves died young and thus recommended they not be purchased. Modern recognition of the health effects of asbestos exposure were initially complicated by the high incidence of tuberculosis in sweatshops in Europe. In 1930, however, Merewether convincingly demonstrated pulmonary fibrosis in asbestos workers, coining the term "asbestosis." In 1955, Doll recognized lung cancer arising in asbestos workers,[13] and in 1960, Wagner et al described 47 cases of mesothelioma in South Africa in a crocidolite (blue amphibole asbestos) mining community, and established the diagnosis (which until then had been debated) as well as both occupational and bystander risk.[14] Asbestos fibers in lung tissue are found as either uncoated fibers or ferruginous bodies, fibers coated by macrophages with iron substance. Because fibers persist once in situ, exposure continues decades after fibers were initially inhaled. Pleural calcified plaques may develop decades after a significant asbestos exposure. Asbestos is not carcinogenic in the Ames test; however, it induces reactive oxygen species and damages DNA, producing chronic inflammation and eventual fibrosis. Because of the pattern of asbestos use, mesothelioma incidence will likely peak in Europe[15,16] in about 2020. Surveillance, Epidemiology, and End Results (SEER) data in the United States show that the incidence peaked in 1994-1995 at 1.2 cases per 100,000, with a small nonsignificant decline to 1.0 cases per 100,000 in 2000-2002.[1] The incidence varies considerably by location, being highest along coastal towns associated with shipyards and in states with industrial asbestos textile mills (Figure 2). At- risk occupations include asbestos miners and millers, but also insulation, shipyard, and maintenance workers,[ 17] as well as auto mechanics.[18] Workers manufacturing cigarette filters were exposed due to asbestos in the filter paper.[19] Incidence is lower for African-Americans than for European- Americans (Figure 1) because of work place patterns favoring whites for employment. Few women were employed in these industry jobs at the time.[1] Presentation About 85% of mesotheliomas arise in the pleura, about 9% in the peritoneum, and a small percentage in the pericardium or tunica vaginalis testis.[1] Peritoneal mesotheliomas may develop more frequently in men with heavier asbestos exposure. Women comprise only about 18% of patients with pleural mesothelioma, but 42% of those with peritoneal mesothelioma. Risk is highest at about age 80 to 84 (Figure 3).[1] The median age in reported clinical trials is frequently a decade or more younger than the median age of patients in the SEER database. Patients with pleural mesothelioma initially complain of shortness of breath or chest pain, whereas those with peritoneal mesothelioma present with increased abdominal girth or abdominal pain.[20,21] Fine-needle biopsy can be used to document recurrence or metastases but is not sufficiently reliable for primary diagnosis.[22] The histology of about half of mesotheliomas is epithelial (tubular papillary), with the remainder being sarcomatous, or mixed.[20,21] Multicystic mesotheliomas and welldifferentiated papillary mesotheliomas are associated with a long survival in the absence of treatment. Thus, these patients should be excluded from clinical trials intended for the usual rapidly lethal histologic variants of mesothelioma.[23-25] Prognostic Variables and Survival The median survival in the Surveillance, Epidemiology, and End Results (SEER) database is approxi mately 7 months and has not improved over the past 2 decades, for either pleural or peritoneal mesotheliomas (Figure 4).[1] Poor-prognostic variables include sarcomatous histology, pleural as opposed to peritoneal primaries, older age, pain at diagnosis, male gender, poor performance status, and perhaps high lactate dehydrogenase (LDH), white blood cell, and platelet levels.[ 20,26] In a recent European Organization for Research and Treatment of Cancer (EORTC) analysis, poor prognosis was associated with a poor performance status, elevated white blood cell (WBC) count, lack of a definitive histologic diagnosis of mesothelioma, male gender, and sarcomatous histologic subtype.[27] In a Cancer and Leukemia Group B (CALGB) multivariate analysis, poor performance, pleural involvement, LDH > 500 IU/L, chest pain, platelets > 400,000/μL, sarcomatous or mixed histology, and age older than 75 years predicted poor survival.[28] Grouping prognostic variables, the group with the best survival (14 months) had a performance status of 0, and were ei- ther younger than 49 years or aged 50 or more with a hemoglobin of 14.6 or more. Those with a performance status of 1 or 2 and WBC more than 15.6/μL had a median survival of only 1.4 months.[28] Surgical and Radiologic Treatment of Localized DiseasePleural MesotheliomaIn the SEER 9 regions from 1987 to 2002, pleural mesothelioma constituted 74% of all mesotheliomas in women and 88% in men.[1] Computed tomography (CT), magnetic resonance imaging (MRI), and 18Ffluorodeoxyglucose (FDG) imaging provide assessment of extent of disease.[29,30] Surgical alternatives include biopsy only, pleurodesis, pleurectomy, and pleuropneumonectomy. Optimal management of localized disease with surgery, radiation, or both is not established. In studies of thoracoscopic talc pleurodesis, the median survival was 7 to 9 months-not significantly different from survivals in the SEER database. After pleurectomy, the median survival ranges from 5 to 20 months. Extrapleural pneumonectomy results in median survivals of 9 to 21 months in various series. Certainly, selection of patients healthy enough to undergo surgery accounts for some or all of this difference. Of 76 patients assessed with contrast- enhanced MRI in Leicester, United Kingdom, 51 underwent extrapleural pneumonectomy or radical pleurectomy/decortication. Pathologic stage was correlated with radiologic staging, with particular emphasis on tumor resectability. On MRI, 17 patients (22%) had unresectable disease that was not visible on CT scan. Fiftyone patients (67%) underwent surgery; pathologic nodal data were incomplete in three who were excluded from further analyses. The median interval from MRI to surgery was 17 days. MRI correctly predicted resectability in 97%. (Two patients had unexpected extensive disease at thoracotomy.) MRI tumor stage was accurate in 48%, and understaged in 50%, largely due to pericardial involvement, which significantly affected neither resectability nor prognosis. Nodal stage was correctly identified in 60% of patients. Thus, MRI is unlikely to contribute significantly to nodal staging, but it remains valuable for selection of patients for surgery.[30] In a Dana-Farber/Brigham and Women's Hospital series (Table 1) of 52 selected patients who received extrapleural pneumonectomy, cyclophosphamide, doxorubicin, and cisplatin (CAP) chemotherapy, and radiotherapy, perioperative morbidity and mortality rates were 17% and 5.8%, respectively. The median survival was 16 months. One- and twoyear survival rates were 77% and 50% for patients with epithelial histology and 45% and 7.5% for those with sarcomatous and mixed variants (P < .01); all of the latter patients died by 25 months. Patients with negative regional mediastinal lymph nodes survived longer than those with positive nodes (P < .01). Of the subset of patients with epithelial histology and negative mediastinal lymph nodes, 45% were alive at 5 years.[31] Of 132 patients with malignant pleural mesothelioma who underwent surgery in a Leicester, UK, study, 53 underwent extrapleural pneumonectomy, and 79, less radical resections. Mortality at 30 days was similar for the two groups (Table 1). Time to disease progression and survival favored extrapleural pneumonectomy, although selection bias may account for the difference. Nodal involvement of N2 nodes compared with N0/1 involvement was associated with shorter survival (197 vs 358 days, P = .02).[32] Radiation series described symptom control in some but not all patients. Conformal radiotherapy is currently under evaluation and would at least theoretically deliver a higher dose to the pleura while avoiding heart and lung tissue. Recent studies of surgery, radiation, or combined-modality therapy are shown in Table 1. After invasive procedures, the risk of tumor masses growing out of needle or incision scars was diminished by radiation in one study[42] but not a second randomized study (Table 2).[43] Peritoneal MesotheliomaThe second most common site for the development of mesothelioma is the peritoneum. Though overall more men than women develop peritoneal mesothelioma, a higher proportion of women than men develop the disease in the peritoneal cavity. In the SEER 9 regions from 1987 to 2002, peritoneal mesothelioma comprised 19% of all mesotheliomas in women and 7% in men. Typically, patients with peritoneal mesothelioma present with increasing abdominal girth, abdominal pain, ascites, fever, and night sweats. CT scan findings are varied and include ascites, localized tumor masses, or diffuse peritoneal involvement. Some long-term survivors have been described in a Dana-Farber Cancer Institute study of surgical debulking, intraperitoneal chemotherapy, and radiotherapy.[51,52] In a National Cancer Institute series, 49 patients underwent laparotomy, tumor resection, hyperthermic intraperitoneal (IP) cisplatin, and postoperative IP fluorouracil and paclitaxel. The median progression-free survival was 17 months and the medi an survival was 92 months. Patients with debulking surgery, superficial tumor, minimal residual disease after resection, and age less than 60 years survived longer.[53] Given patient selection, the contribution of any of these modalities to prolonged survival is unknown. Pericardial MesotheliomaPericardial mesothelioma is a rare but lethal malignancy for which treatment options are limited.[54,55] Patients present with symptoms of constrictive pericarditis. MRI may demonstrate the extent of infiltration of the myocardium or cardiac vessels. Misdiagnosis is common, with the correct diagnosis frequently made at surgery or autopsy. Occasionally, patients survive disease-free after complete resection, but myocardial invasion usually precludes complete resection. Surgery is often useful to relieve effusions, pericardial tamponade, or constriction. Chemotherapeutic regimens established in pleural mesotheliomas, as well as intracavitary chemotherapy or irradiation and photodynamic therapy treatments, have been reported with modest benefit.[54,55]